The invention relates to a radar apparatus, and more particularly, to a radar apparatus that receives a reflected electric wave of electric waves transmitted from a vehicle, which is reflected from a target, by a plurality of receiving antennae and thus detects a position of the target.
There is a radar apparatus which measures a distance between a vehicle (own vehicle) that a driver drives and a preceding vehicle or front obstacle (target) or a target such as vehicle approaching from the rear and a direction thereof all the time and thus prevents a collision and performs an automatic traveling. In the radar apparatus, an electrical wave is transmitted from an antenna provided to the own vehicle, a reflected wave, which is reflected from the target, is received by the antenna, signal processing is performed for a signal obtained by the reception, an arrival direction of the reflected wave is estimated and thus the target is detected. As methods of estimating the arrival direction of the reflected wave, there are a DBF (Digital Beam Forming) method, a Capon method, an LP (Linear Prediction) method, a minimum norm method, a MUSIC (Multiple Signal Classification) method, an ESPRIT (Estimation of Signal Parameters via Rotational Invariance Techniques) method and a PRISM (Propagator method based on an Improved Spatial-smoothing Matrix) method.
The in-vehicle radar apparatus detects even a target in an area beyond a detection object area when the transmitted electric wave (beam) is enlarged. For example, when a traveling traffic lane on which the own vehicle travels is set as the detection object area, since even an obstacle beyond the traveling traffic lane is detected, it is necessary to determine whether the detected obstacle is an obstacle on the traveling traffic lane. Accordingly, an obstacle detection apparatus for vehicle mount disclosed in Patent Document 1 includes a distance measurement means that transmits an electric wave and thus measures a distance to an object and a received electric wave strength detection means that detects a received electric wave strength of the electric wave reflected at the object, and determines whether the detected object is a vehicle to be originally detected or an obstacle beyond the traveling traffic lane from a change in the strength of the received electric wave in accordance with the measured distance.
The in-vehicle radar apparatus disclosed in Patent Document 1 is an apparatus that receives the electric wave reflected at the obstacle by one antenna. Compared to this, an electronic scan radar (for example, refer to Patent Document 2) has been known which receives a transmission electric wave transmitted toward the front of the own vehicle and an electric wave reflected from a target by a plurality of antennae and calculates an angle of the target (an angle indicating how much the target is deviated from the traveling direction of the own vehicle) from a phase difference between the obtained received electric waves. In the electronic scan radar, the target detection is performed by detecting the phase difference, which is performed by using the plurality of receiving antennae, within a range of ±180°.
Patent Document 1: JP 2005-69739A (FIGS. 1 to 4)
Patent Document 2: JP 2006-91029A (FIGS. 1 and 3)
In the electronic scan radar using the receiving antennae, when the reflected wave from the target is received to estimate the arrival direction of the electric wave, the detection range of the phase difference is ±180°. Accordingly, when an electric wave having a phase difference of 360° or larger from the transmission electric wave, for example an electric wave having a phase difference of 420° is received, the corresponding electric wave is erroneously detected as the received electric wave having a phase difference of 60° (which is obtained by subtracting 360° from 420°), so that the angle of the target is erroneously detected.
In order to solve the above problem, a method of narrowing an interval of the receiving antennae (i.e., widening an angle range to be detected) may be considered. However, when the interval of the antennae is narrowed, an interference of the received electric waves between the receiving antennae is caused, so that power and accuracy of the phase are lowered. As a result, the angle estimation accuracy, which is required for the radar apparatus, is reduced, so that the resolution capability cannot be satisfied.